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PCM5102A: Poor THD+N test result...

Peter Tseng
Genius 5555 points
Part Number: PCM5102A

Dear Sirs,

Atached is the PCM5102A 1KHz line out FFT/THD+N report.

In the FFT report, there are harmonic tones at 2k, 3k, and 5k and the noise floor is large.

May I know how to tune PCM5102A to fix and improve the test performance.

Thanks.

  • 0
    TI__Guru 52683 points

    Hi Peter,

    Note that the THD+N test is limited to -1dBFS.  I cannot make out the amplitude of your input signal in the screen shots.  

    In addition, you should verify that the your supplies AVDD/DVDD/CPVDD are at the nominal +3.3V.  You can also verify that VNEG is about -3.3V to -3.0V.  You should verify the 2.2uF caps on VNEG and the flying cap pins is rated for at least 10V.  

    You should also verify if you have the low pass filter on the output.  

  • 0 in reply to Paul_Frost
    Genius 5555 points

    Hi Sir,

    Here are the feedback from customer.

    In addition, you should verify that the your supplies AVDD/DVDD/CPVDD are at the nominal +3.3V. 

    -> normal +3.3V

    You can also verify that VNEG is about -3.3V to -3.0V.

    -> Keep 3.3V.

    You should verify the 2.2uF caps on VNEG and the flying cap pins is rated for at least 10V.  

    -> will change to 10V voltage rate from 6.3V.

    1.  Is there any problem with customer test result?

    2. The VREG is 3.3V, is that correct or not?

    3. Any suggestion to fix the harmonic tone of 2K, 3K, and 5KHz

    4. Customer schematics was attached.

    5. What is the test step and config in TI for SNR test of PCM5102A?

    6. Is there any 16 bits SNR test report in TI?

    Thanks.

    PCM5102A.PDF

  • 0 in reply to Peter Tseng
    TI__Guru 52683 points

    Hi Peter,

    I still would like them to confirm that they are using a -1dBFS input for this test.  If they are using beyond that, then you might see some clipping which could result in extra harmonics.

    You could also verify the supplies when generating this test tone.  If the charge pump cannot generate the voltage, it will start to clip.

    You can also verify the filters are valid.

    Generally, we measure SNR by applying a -60dBFS signal and do a THD+N measurement.  We then add +60dB to that result to give us the SNR value.

  • 0 in reply to Paul_Frost
    Genius 5555 points

    Hi Paul,

    Here are customer test result for -1dBfs(1.26Vpeak) and -60dBfs(1.1mVpeak) about PCM1502A EVM and customer board.

     -1dBfs for EVK:

    -1dBfs for customer board:

    -60dBfs for EVK:

    -60dBfs for customer board:

    For -1dBfs input, there is 3rd harmonic tone in customer board, but no in EVK.

    For -60dBfs input, EVM and customer board test result are similar.

    Is there any suggestion for layout or components to improve the harmonic tone issue?

    Thanks.

  • 0 in reply to Peter Tseng
    TI__Guru 52683 points

    Hi Peter,

    The schematic you shared only had the PCM5102A - is there any additional circuitry connected to the outputs? Are they buffering or amplifying the outputs?

    In regards to layout recommendations: I recommend that they have a direct ground return path for the DAC outputs.  While this device is single-ended, there is still current that returns to the ground, so it is useful to route the outputs differentially with a ground signal as well.

    Thanks,
    Paul